1. Field of the Invention
The present invention relates to a one-pack type epoxy resin composition. More particularly, the present invention relates to a one-pack type epoxy resin composition which has high storage stability, which can be made into a film, which is curable at relatively low temperatures, and which has high adhesivity.
2. Description of the Prior Art
The epoxy resin composition used mainly as an adhesive or, in some cases, as a molding material includes (1) a two-pack type epoxy resin composition wherein an epoxy resin (a main component) and a curing agent are prepared separately and, when the composition is used, are mixed and (2) a one-pack type epoxy resin composition which is a mixture of an epoxy resin (a main component) and a curing agent.
In the two-pack type epoxy resin composition, the epoxy resin (a main component) and the curing agent must be accurately measured and mixed when the composition is used; therefore, a measurement error is apt to occur, and the quick reaction between the epoxy resin and the curing agent allows no storage after mixing of the two components.
Meanwhile, in the one-pack type epoxy resin composition which is a mixture of an epoxy resin (a main component) and a curing agent, there are no problems such as mentioned above; however, commencement of a reaction of the epoxy resin with the curing agent and subsequent curing, and resultant reduction in the storage stability of the composition are anticipated. Therefore, various proposals have been made to suppress the reaction of the epoxy resin with the curing agent when the composition is in storage.
As one-pack type epoxy resin compositions having high storage stability, there is generally known a one-pack type epoxy resin composition using dicyandiamide as the curing agent; and there is also known a one-pack type epoxy resin composition using an imidazole as the curing agent.
Also, as one-pack type epoxy resin compositions having good storage stability, there are known, for example, a composition comprising an epoxy resin and a phenol having at least two functional groups (JP-A-8-183835); a composition comprising an epoxy resin and a carbodiimide (JP-A-5-320611); and a composition comprising an epoxy resin, a curing agent (e.g. dicyandiamide, phenol or acid anhydride) and guanidine as a curing accelerator (JP-A-60-28424).
The above proposals, however, have respective problems. The dicyandiamide has a high melting point of 200.degree. C. or higher and therefore must be dispersed in an epoxy resin in a solid state, and the resulting epoxy resin composition inevitably gives rise to nonuniform curing; when the dicyandiamide is used by being dissolved in a solvent, the resulting composition gives rise to uniform curing but has inferior storage stability.
The imidazole has high reactivity and gives an epoxy resin composition of very low storage stability when having a low melting point and, when having a high melting point, gives a composition of improved but still insufficient storage stability and moreover has the same problems as the dicyandiamide does.
Meanwhile, the one-pack type epoxy resin composition comprising an epoxy resin and a phenol and the one-pack type epoxy resin composition comprising an epoxy resin and a carbodiimide have good storage stability as mentioned in the above-mentioned literatures but require a relatively high temperature of 180.degree. C. for curing.
Further, the one-pack type epoxy resin composition comprising an epoxy resin, a curing agent (e.g. Dicyandiamide and the like) and guanidine as a curing accelerator, similarly to the above compositions comprising a phenol or a carbodiimide, requires a relatively high temperature for curing; moreover, the composition has no film-formability and cannot be handled as a film-shaped composition.